This invention relates to a variable capacitance diode device applicable to high frequency use and used as a tuning device. More particularly, the invention relates to a variable capacitance diode device with a high quality factor Q.
FIGS. 1A and 1B show a conventional variable capacitance diode device. In these figures, a semiconductor body is composed of a semiconductor substrate (N.sup.++) 10 of N-conductive type with high impurity concentration and a low specific resistance and an epitaxial layer (N.sup.--) 20 of N-conductive type with a relatively high specific resistance formed on the semiconductor substrate 10.
A diffusion layer (N.sup.+) 30 of N-conductive type with high impurity concentration is diffused from a main surface of the semiconductor body to the epitaxial layer 20 so as to reach the semiconductor substrate 10.
Next, a diffusion layer (P.sup.++) 40 of P-conductive type with high impurity concentration is diffused to form a main PN junction J.sub.1 with the diffusion layer 30.
A silicon dioxide film, produced in a diffusion process of the diffusion layers 30 and 40, covering a central main surface portion of the device is removed. Also, a silicon dioxide film 50 covering a PN junction J.sub.2 also formed by diffusing the diffusion layer 40, exposed to the main surface of the semiconductor body, is allowed to remain as a protection film.
A conductor film 60 of e.g. aluminium is formed on the main surface-exposed portion of the semiconductor body having the silicon dioxide film 50. Wires 70 of gold are then bonded to the conductor film 60 to form the variable capacitance diode device.
When the conventional variable capacitance diode device is used at a high frequency, the area of an outer peripheral wall of the diffusion layer 30, which is diffused in a column-shape for obtaining a required maximum capacitance C.sub.max, should be extremely small.
Convergence of electrical field thus occurs due to the skin effect at the high frequency at the area of an outer peripheral wall of the diffusion layer 30. This leads to a high frequency-series resistance which is large and which lowers the quality factor Q.
Furthermore, if the PN junction J.sub.2 of the diffusion layers 30 and 40 is formed not so deep in order to have a larger capacitance-variation ratio, the PN junction J.sub.2 is sometimes destroyed due to the stress of the pressure of a capillary (not shown) when wires 70 are bonded by thermocompression bonding. This degrades the reliability of the diode device.
A junction area of a variable capacitance diode device for high frequency use is made extremely small to satisfy recent demands for the higher quality factor Q and smaller maximum capacitance C.sub.max.
For example, a variable capacitance diode device used in a radio for AM broadcasting has an approximate maximum capacitance C.sub.max in the range of 350 to 650 pF. Also, in UHFTV band, a maximum capacitance C.sub.max approximately in the range of 0.1 to 15 pF is required. This causes the junction area to be extremely small.
On the other hand, with the variable capacitance diode device for high frequency use, the area of the conductor film 60 for wire bonding should be extremely large with respect to the junction area.
Usually, each size of wire-bonded portions 71 of the wires 70 to the device becomes about 3 to 5 times the diameter of each wire 70. The diameter of each wire-bonded portion 71 thus becomes 75 to 125 .mu.m if that of the wire 70 is 25 .mu.m. Therefore, the area of the conductor film 60 is made extremely large so that the wire-bonded portions 71 do not protrude therefrom. This causes unnecessary floating capacitance to be produced between the conductor film 60 extended from the PN junction J.sub.2 like a hook and the epitaxial layer 20. This degrades the high frequency characteristics of the device.